1. Field of the Invention
The present invention is directed to an analog electronic switch circuit with reduced current leakage when the voltage on the input or output of the switch goes either below ground level voltage or above positive rail voltage.
2. Description of the Related Art
Electronic switches in integrated circuits (“IC”) are commonly constructed with Complementary Metal Oxide Semiconductor transistors (“CMOS”) in which an n-channel transistor and a p-channel transistor are directly coupled at their input and output terminals. The switch is controlled by a drive signal applied to both transistor gates in a complementary fashion to make both transistors ON or OFF simultaneously.
FIG. 1 shows a double-diffused CMOS substrate 10 with an n-well region 16 and a p-well region 14 isolated from the substrate 10. These well regions may be the bodies of P-channel and N-channel transistors of a CMOS analog switch. FIG. 2 shows a conventional CMOS transmission gate analog switch 28, with complementary switch transistors 22 and 24 as described above and complementary drive signals 40 and 26 for their respective gates. The body terminals 34 and 36 of switch transistors are connected to their respective voltage rails, as shown in FIG. 2.
In IC's which contain CMOS electronic switches, unintended parasitic bipolar transistors may be formed due to the interaction of the p-n junctions which are in close vicinity of each other. Such a parasitic transistor 17 is shown in FIG. 1. The switch functions correctly when the input voltage is between ground and high voltage supply levels. That is, the switch is in open or closed state depending on the state of the drive signals and little or no unintended current leakage occurs. However, the parasitic transistors can turn on and conduct under certain conditions, which results in undesirable current leakage. Conditions under which current leakage takes place occur when the input voltage on the switch input terminal goes beyond the negative or positive rail voltages, that is, when the input voltage becomes more negative than ground or exceeds the positive supply voltage, thus causing conduction in the parasitic bipolar transistors. In FIG. 2, the input terminals of the transistors 22 and 24 are connected to each other to form a common input terminal 32 and the outputs are connected to each other to form a common output terminal 30. A switch signal on line 38 is input to the gate of 24 and via inverter 20, to the gate of 22 to enable or disable the switch for passing the input signal to the output terminal 30.
In the prior art, a number of approaches have been taken to prevent or minimize this leakage current. For example, U.S. Pat. Nos. 6,218,707 and 5,834,826 provide proposed solutions to this problem, both of which are incorporated herein by reference.